In stope mining drilling is carried out at an upper level while mucking, or removal of the mined rock, is carried out at a lower level in the mine. These levels are connected by raised bores down which mined ore or backfill are passed as appropriate. There are periods of time, therefore, when both he stope cap and mucking brow are open which provides an alternate by-pass route for ventilation air which could be better used in the main drifts of the mine. In some cases reverse air flows can occur. It is, therefore, prudent practise to limit the period of time the brow is left open and also to provide ventilation curtains in the appropriate stops access. Ventilation curtains are, however, very expensive to build and maintain and in any event they frequently lock and cannot withstand the shock wave created by underground blasting operations. Many attempts to circumvent the problems encountered with ventilation curtains have been made with only limited success. Such attempts include foam barriers, which do not stand up to the rigours of underground mining operations; flat steel raise-bore covers which can be covered with loose muck to hold them in place but which are only satisfactory as permanent installations, require smooth surfaces to make an adequate seal and which are susceptible to movement due to concussion shock; and inflatable balloon plugs rather like a navigational or mooring buoy which can be inflated to about 54" diameter to seal against the walls of a circular hole. Balloon plugs are effective as if they are moved by blast they settle back into place, but they suffer from the disadvantage that they do not prevent inadvertent access to the raise nor do they safely cover a fill hole and thus require additional barricading. There is, therefore a considerable need for an inexpensive raise bore cover which can be used independently of ventilation curtains or the like, which can be quickly and easily installed and equally quickly be removed or which can be permanently sealed into place.